Vacuum conveyors transport material (powder, granulate, dust, tablets, small parts, etc.) using a stream of suction gas, usually air or an inert gas.
The material is suctioned from a feeding point, e.g., with a hand suction pipe, feeding hopper, or the like, transported through a tube or pipeline, and led into the vacuum conveyor by means of a suction opening. In the vacuum conveyor, an inserted filter element separates the transported material from the suction gas, usually supported by centrifugal force and sedimentation. The transported material is collected in the trap container of the vacuum conveyor. The filtered suction gas stream leaves the trap container and flows through the vacuum pump driving the conveyor process, which is typically tightly installed directly on the top part of the vacuum conveyor and is to be protected from contamination, independently of decreasing the environmental impact. After a successful filling process of the trap container, the vacuum pump is turned off or the suction gas stream is interrupted by a valve on the filtered (pure) gas side between the trap container and the vacuum pump.
In a standard vacuum conveyor, a discharge valve arranged at the bottom in the trap container is then opened and the transported material falls out of the trap container through the discharge opening.
Bridge-forming materials can be forced out of the trap container by fluidization or excess pressure aids. The adhered filter cakes in the filter are washed by means of a counter airflow outwards from the pure gas side. The duration of the individual suction and discharge cycles is typically regulated by means of cycle control with adjustable suction and discharge times. Suction and discharge times are typically relatively short for vacuum conveyors and usually equal only a few seconds each. Because the filter cakes can be washed quite frequently, this short cycling allows for an extremely compact structural shape with extremely small filter surfaces (in comparison with flow rates in typical air filter systems).
The high flow rates and the alternating load direction, which usually changes several times per minute (suction, counter flow pulse), set strict requirements on filter systems for vacuum conveyor devices. The filter system solutions known from the prior art are often not designed for these requirements. These solutions include, in particular, so-called filter cartridges made from sintered plastics, metals, or ceramic materials, and so-called deep-bed filters, which are shaped as rigid pipes closed on one end.
The suctioned gas passes the pores of the filter, while the transported material or the fine material portion adhered to the filter is stopped on the surface of the filter and embedded at a certain depth in the pores of the filter. The result is an increased differential pressure in the filter, which can lead to the appearance of fatigue in the filter due to the frequently changing forces and ultimately to filter breakdown. The fineness of this filter typically equals 1-1.5 μm for vacuum conveyors.
In addition, one-stage surface filters made from flexible planar material are known. They usually operate with a very fine filter fabric or filter fleece with an attached membrane layer, which is also in the position to retain very fine particles (for materials >0.1 μm). However, in vacuum conveyors, this fine filter layer is exposed to high loads, like extremely frequent impact of fast particles, so that membranes can be destroyed relatively quickly. These filter materials are typically folded in the form of a star like automotive air filters, in order to achieve a large filter surface area for low flow rates.
Finally, so-called bag filters are usually made from a felt material, which is often also coated on the flow side with a membrane. Bag filters offer the advantage that they inflate during the counter-airflow filter cleaning and therefore the filter cakes can be washed much better than in other filter systems. Their advantage is simultaneously their disadvantage: the frequent movement of the filter bag leads to large loads on joints, connection points, and the filter membrane.
In order to guarantee that the environmental impact due to residual particles in the already filtered air is kept below a permissible low level also for critical materials, e.g., that are dangerous to human health, the exhaust air from vacuum conveyors is typically forwarded to an exhaust-air cleaning device with only single-stage filters. Alternatively, for further cleaning of the exhaust air of the vacuum conveyor, a so-called police filter can be used, which guarantees the quality of the exhaust air as a second filter stage. This second filter stage can also be used as an additional module downstream of the actual working filter but still before the vacuum pump of the vacuum conveyor. However, in many cases this results in problems in the structural height.